Recently developed thermal imaging systems typically include (IR) detectors coupled to a (CCD) by way of a direct injection method. A multitude of these detectors and input gates together with their respective charge coupled devices (CCD) form a device commonly known as a focal plane array. In response to thermal radiation, the (IR) detectors in the array, each produce a photo current signal which is applied to a respective source node of the direct injection input structure to which each detector is connected. The source's node input impedance and the internal shunting resistance across each IR photodetector, share the detector's photocurrent and act as a sink for such a photocurrent. When the voltage applied to the direct injection gate reaches a certain value referred to as a threshold value, photocurrent begins to flow into the CCD through the source input node of the direct injection structure. The bias voltage required across each IR detector is small, and is equal to the difference between the voltages applied to the direct injection gate minus the threshold voltage (V.sub.T), and minus the voltage applied to detector nodes not connected to the input source node. In large focal plane arrays the direct injection input gates are all tied to a common control line; and those detector nodes which are not connected to the input source node are coupled to a common control line. For a large number of detectors coupled to a CCD, the bias voltage developed across each detector is a direct function of the threshold voltage associated with the particular direct injection structure to which the detectors are connected. The difficulty in maintaining a uniform threshold value increases substantially with array size. In the event that a uniform threshold value is not maintained within certain limits, the focal plane array responds, such that those direct injection structures with the lowest threshold value "turn on" first, and the direct injection structures with the higher threshold voltage value, remain "off". Upon further increase, a sufficient voltage will be applied to the common direct injection gate to "turn on" all injection structures. This results in some detectors being biased above the value desired for proper operation. Thus, some CCD potential wells may be saturated as a result of the extra bias applied across some IR detectors before other detectors are sufficiently biased, for "turning on" and inject photocurrent into their respective input structure. This saturation, of course, is deleterious to the operation of the array, including the possible loss of signal information on the entire output channel.
Recent studies indicate that the control of threshold uniformity may be obtained by controlling the uniformity of certain process variables in the manufacture of CCD devices. Uniformity of the oxide thickness and the density variation of the interface state between the substrate and the overlying oxide layer, have a substantial affect on threshold uniformity of a focal plane array.
In one exemplary process, the gate oxide thickness (t.sub.ox) averaged 1130 Angstroms; and the deviation in oxide thickness of such a layer was from 5 Angstroms within each wafer to 15 Angstroms within each run of wafers, and 34 Angstroms between different runs. This deviation in oxide thickness resulted in a deviation of approximately 23 millivolts across a single 21/2 inch wafer, a deviation of 40 millivolts between wafers within a single run, and a deviation of 165 millivolts between individual process runs. With respect to the deviations in interface state density (N.sub.ss) between the substrate and the oxide layer, a recent analysis indicates that the interface state density contributes a deviation of 17.5 millivolts to threshold uniformity while uniformity of the oxide thickness contributes only 41/2 millivolts deviation to the threshold voltage.
The remaining process variables, such as deviation in oxide charger (Q.sub.o), implant doping (N.sub.I) for adjusting the threshold level, deviations of the doping level of the polysilicon gate (N.sub..epsilon.), and doping level deviations of the substrate (N.sub.B) all affect threshold uniformity, but to a much lesser extent. In summary, as the process parameters vary in a focal plane array, the threshold voltages vary from one detector to another. A more detailed explanation of threshold variation effects in IR detector/CCD focal plane arrays is included in a publication prepared for the Office of Naval Research in Arlington, VA, by the Environmental Research Institute of Michigan, dated August 1978.
Heretofore, in an attempt to obtain threshold voltage uniformity, input circuits were designed with a feedback loop for the purpose of realizing uniform threshold input gate voltages, at the expense of a substantially larger cell size, and a more complex focal plane array. Improved threshold control through improved uniformity of the process variables, particularly the gate oxide thickness and interface state density, which cause the greatest deviation in threshold voltage, at the present time, is impractical to obtain.
Therefore, it is desirable to provide an improved CCD focal plane array that exhibits substantial uniformity in its threshold voltage values across the entire array without the necessity of attempting to establish and/or develop a high degree of uniformity in the process variables.